1894.] 



on the Work of Hertz. 



339 



filings good. I have taken rough measurements of the resistance, by 

 the simple process of restoring the original galvanometer deflection 

 by adding or removing resistance coils. A J-inch tube 8 inches 

 long of selected iron turnings, Fig. 18, had a resistance of 2500 

 ohms in the sensitive state. A feeble stimulus, caused by a distant 

 electrophorus spark, brought it down 400 ohms. A rather stronger 

 one reduced it by 500 and 600, while a trace of spark given to a 

 point of the circuit itself, ran it down 1400 ohms. 



This is only to give an idea of the quantities. I have not yet 

 done any seriously metrical experiments. 



From the wall diagram which summarises the various detectors, 



Fig. 19. 



Oz^l 



Radiator used in the library of the Royal Institution, exciting the Coherer 

 (Fig. 17) on the lecture table in the theatre. (Sphere 5 inches diameter.) 



and which was prepared a month or so ago, I see I have omitted 

 selenium, a substance which in certain states is well known to 

 behave to visible light as these other microphonic detectors behave 

 to Hertz waves. It is now inserted, but with a query to indicate 

 that its position in the table is not certainly known. 



And I want to suggest that quite possibly the sensitiveness of 

 the eye is of the same kind. As I am not a physiologist I cannot 

 be seriously blamed for making wild and hazardous speculations in 

 that region. I therefore wish to guess that some part of the retina 

 is an electrical organ, say like that of some fishes, maintaining an 

 electromotive force which is prevented from stimulating the nerves 



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